THE EFFECTS OF α‐ADRENOCEPTOR ANTAGONISTS ON CONTRACTILE RESPONSES TO 5‐HYDROXYTRYPTAMINE IN DOG SAPHENOUS VEIN

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Abstract
1 5-Hydroxytryptamine (5-HT) contracted isolated saphenous vein strips of the dog, producing a biphasic concentration-effect curve. The first phase occurred with low concentrations of 5-HT (1.0 × 10−8 to 5.0 × 10−6mol/1) with a plateau between 1.0 × 10−6mol/1 and 1.0 × 10−5mol/1. The second phase occurred with high concentrations of 5-HT (greater than 1.0 × 10−5 mol/1). 2 The α-adrenoceptor antagonists, phentolamine (5.0 × 10−8 to 5.0 × 10−7 mol/1), labetalol (1.0 × 10−6 to 1.0 × 10−5 mol/1) and thymoxamine (1.0 × 10−6 to 1.0 × 10−5 mol/1), antagonized responses to high concentrations of 5-HT but responses to low concentrations of 5-HT were not antagonized. 3 The effects of high concentrations of 5-HT were antagonized by cocaine (1.0 × 10−6 to 1.0 × 10−5 mol/1) and were not evident in veins removed from dogs pretreated with syrosingopine. 4 It is concluded that in the saphenous vein, low concentrations of 5-HT act directly on specific 5-HT receptors and that high concentrations of 5-HT also act indirectly on α-adrenoceptors by displacing noradrenaline from neuronal stores.